1. Field of the Invention
The invention relates to multilayer fluoropolymer-containing films having improved interlayer adhesion. More particularly, the invention pertains to multilayer films including a fluoropolymer layer attached to a thermoplastic polymer layer via an intermediate poly(ester-urethane) copolymer containing adhesive layer, articles produced therefrom, and products which are encased by the multilayer films.
2. Description of the Related Art
A wide variety of thermoplastic polymers and films formed from such thermoplastic polymers are known. Important physical characteristics of such films include its barrier properties, including barriers to gas, aroma, and/or vapor such as water vapor, as well as its physical characteristics, such as toughness, wear and weathering resistances, and light-transmittance.
It is well known in the art to produce multilayer fluoropolymer-containing films. See, for example, U.S. Pat. Nos. 4,146,521; 4,659,625; 4,677,017; 5,139,878; 5,855,977; 6,096,428; 6,138,830; and 6,197,393. Many fluoropolymer materials are commonly known for their excellent moisture and vapor barrier properties, and therefore are desirable components of packaging films, particularly lidding films and blister packages. In addition, fluoropolymers exhibit high thermal stability and excellent toughness. However, such use of fluoropolymers is restricted to specialty packaging applications due to their relatively high cost. A suitable means of reducing the cost of a packaging material fabricated from a costly polymer is to form multilayer structures in which the polymer film is laminated with other, less costly polymer films. This approach is particularly desirable for the fluoropolymer packaging applications since a thin layer of the fluoropolymer is often all that is needed to take advantage of the desirable properties of the fluoropolymer while minimizing the cost. However, fluoropolymers do not adhere strongly to most other polymers. In fact, most fluoropolymers are known for their non-stick characteristics. This is very disadvantageous, because poor bond strength between layers can result in the delamination of multilayer films.
To improve the bond strength between a layer of a fluoropolymer and a layer of a thermoplastic polymer (e.g. a non-fluoropolymer layer), an adhesive layer may be used between adjacent layers. For example, U.S. Pat. No. 4,677,017 discloses coextruded multilayer films which include at least one fluoropolymer film and at least one thermoplastic film which are joined by the use of an adhesive polymer, particularly ethylene/vinyl acetate polymers, as an adhesive layer. U.S. Pat. No. 4,659,625 discloses a fluoropolymer multilayer film structure which utilizes a vinyl acetate polymer adhesive layer. U.S. Pat. No. 5,139,878, discloses a fluoropolymer film structure using an adhesive layer of modified polyolefins. U.S. Pat. No. 6,451,925 teaches a laminate of a fluoropolymer layer and a non-fluoropolymer layer using an adhesive layer which is a blend of an aliphatic polyamide and a fluorine-containing graft polymer. Additionally, U.S. Pat. No. 5,855,977 teaches applying an aliphatic di- or polyamine to one or more surfaces of a fluoropolymer or non-fluoropolymer material layer.
In addition to traditional fluoropolymer packaging applications, fluoropolymer-containing films are presently employed as barrier films in the electroluminescent lamp (EL) industry, where the films are used to encapsulate sensitive luminescent materials. Electroluminescence is a means of generating light by the electrical excitation of light emitting phosphors. In a typical electroluminescent lamp, light emitting phosphors are located between two electrically conductive electrodes, wherein one is opaque and the other is transparent to allow light to escape. When an alternating current is applied to the plates, the phosphors are excited and produce light. As an integral part of an electroluminescent lamp assembly, multilayer barrier films are commonly used to encase or encapsulate the functioning electrode-phosphor structure, providing improved structural properties and allowing the lamps to be used under extreme environmental and temperature conditions. Due to their excellent moisture and vapor barrier properties, and their high thermal stability and excellent toughness properties, fluoropolymer-containing films are particularly effective encapsulating materials.
There is a continuing need in the art for further improvements in fluoropolymer-containing multilayer films and film structures. For example, it is well known to produce electroluminescent devices by coating a polyethylene terephthalate (PET) substrate with conductive materials, followed by encapsulation with a barrier film. As technology developed, the advantages of fluoropolymer films were recognized and barrier films including polychlorotrifluoroethylene (PCTFE) were introduced. These early barrier films for EL applications used a PCTFE film with an extrusion coating of an ethylene acrylic acid (EAA) copolymer. However, EAA films have a less than desired bond strength when thermal bonded to internal PET layers of electroluminescent lamps. Further, EAA has a high sealing temperature of about (300° F.) 148.9° C., which requires additional time and energy for processing.
Accordingly, there is a particular need in the art of electrolunminescent lamps for improved encapsulation materials.
The present invention provides a solution to this need in the art. The invention provides multilayer films comprising a fluoropolymer layer and a thermoplastic polymer layer which are attached by an intermediate adhesive layer comprising a poly(ester-urethane) copolymer. It has been found that a poly(ester-urethane) copolymer has exceptional adhesion to both fluoropolymer layers and non-fluoropolymer layers. In the preferred embodiment of the invention, the thermoplastic polymer layer comprises an ethylene vinyl acetate (EVA) copolymer. Ethylene vinyl acetate provides significant benefits over EAA. First, EVA seals at about 250° F. (121° C.), allowing for increased productivity. Second, EVA forms a significantly better thermal bond with PET than EAA. Third, EVA is crystal clear at room temperature, allowing it to be used for traditional packaging applications. Also, with the addition of optional ultraviolet light absorbers, EVA protected EL products are acceptable for outdoor applications. The multilayer films are also highly effective for use as protective packaging films, such as archival bags.